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Thread: Query ending up with hitting all the partition with sub-query in the projection list

Query ending up with hitting all the partition with sub-query in the projection list

From
Rushabh Lathia
Date:
Hi All,

Query with the sub-query in the projection list ending up with hitting all the
partition table even though having proper partition key condition.

Example:

CREATE TABLE measurement (
    city_id         int not null,
    logdate         date not null,
    peaktemp        int,
    unitsales       int
);

CREATE TABLE measurement_y2006m02 (
    CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
) INHERITS (measurement);
CREATE TABLE measurement_y2006m03 (
    CHECK ( logdate >= DATE '2006-03-01' AND logdate < DATE '2006-04-01' )
) INHERITS (measurement);

CREATE TABLE test (
    a   int,
    b   date);

-- Hitting all the partition table
postgres=# explain select a , ( select city_id from measurement where logdate = test.b and logdate = '2006-02-02') xyz from test;
                                                QUERY PLAN                                                 
-----------------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..206289.95 rows=2140 width=8)
   SubPlan 1
     ->  Result  (cost=0.00..96.38 rows=27 width=4)
           ->  Append  (cost=0.00..96.38 rows=27 width=4)
                 ->  Result  (cost=0.00..32.13 rows=9 width=4)
                       One-Time Filter: ('2006-02-02'::date = test.b)
                       ->  Seq Scan on measurement  (cost=0.00..32.13 rows=9 width=4)
                             Filter: (logdate = test.b)
                 ->  Result  (cost=0.00..32.13 rows=9 width=4)
                       One-Time Filter: ('2006-02-02'::date = test.b)
                       ->  Seq Scan on measurement_y2006m02 measurement  (cost=0.00..32.13 rows=9 width=4)
                             Filter: (logdate = test.b)
                 ->  Result  (cost=0.00..32.13 rows=9 width=4)
                       One-Time Filter: ('2006-02-02'::date = test.b)
                       ->  Seq Scan on measurement_y2006m03 measurement  (cost=0.00..32.13 rows=9 width=4)
                             Filter: (logdate = test.b)
(16 rows)

-- With swapping the condition hitting only one partition
postgres=# explain select a , ( select city_id from measurement where logdate = '2006-02-02' and logdate = test.b ) xyz from test;
                                                QUERY PLAN                                                 
-----------------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..137537.10 rows=2140 width=8)
   SubPlan 1
     ->  Result  (cost=0.00..64.25 rows=18 width=4)
           ->  Append  (cost=0.00..64.25 rows=18 width=4)
                 ->  Result  (cost=0.00..32.13 rows=9 width=4)
                       One-Time Filter: (test.b = '2006-02-02'::date)
                       ->  Seq Scan on measurement  (cost=0.00..32.13 rows=9 width=4)
                             Filter: (logdate = '2006-02-02'::date)
                 ->  Result  (cost=0.00..32.13 rows=9 width=4)
                       One-Time Filter: (test.b = '2006-02-02'::date)
                       ->  Seq Scan on measurement_y2006m02 measurement  (cost=0.00..32.13 rows=9 width=4)
                             Filter: (logdate = '2006-02-02'::date)
(12 rows)

Here if with the swap of sub-query WHERE clause logdate = test.b and logdate = '2006-02-02' to 
logdate = '2006-02-02' and logdate = test.b query hitting proper partition.


Any input/comments ?

Regards,
Rushabh Lathia


Re: Query ending up with hitting all the partition with sub-query in the projection list

From
Tom Lane
Date:
Rushabh Lathia <rushabh.lathia@gmail.com> writes:
> *-- Hitting all the partition table*
> postgres=# explain select a , ( select city_id from measurement where *logdate
> = test.b and logdate = '2006-02-02')* xyz from test;

Hm.  What's happening here is that you get an equivalence class
containing logdate, test.b, and '2006-02-02', where test.b is actually
a Param supplied from the outer query level.  So the equivclass.c
machinery considers that both test.b and '2006-02-02' are constants,
and it just picks the first one to construct derived equalities from.
So what comes out is "test.b = logdate and test.b = '2006-02-02'", and
neither of those can be used by the constraint-exclusion machinery to
prove that some partitions of "measurement" can be skipped.

What we need is to teach generate_base_implied_equalities_const() to
prefer an actual constant to pseudo-constants.  Will fix, thanks for the
example!
        regards, tom lane

Re: Query ending up with hitting all the partition with sub-query in the projection list

From
Rushabh Lathia
Date:


On Fri, Oct 26, 2012 at 10:39 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Rushabh Lathia <rushabh.lathia@gmail.com> writes:
> *-- Hitting all the partition table*
> postgres=# explain select a , ( select city_id from measurement where *logdate
> = test.b and logdate = '2006-02-02')* xyz from test;

Hm.  What's happening here is that you get an equivalence class
containing logdate, test.b, and '2006-02-02', where test.b is actually
a Param supplied from the outer query level.  So the equivclass.c
machinery considers that both test.b and '2006-02-02' are constants,
and it just picks the first one to construct derived equalities from.
So what comes out is "test.b = logdate and test.b = '2006-02-02'", and
neither of those can be used by the constraint-exclusion machinery to
prove that some partitions of "measurement" can be skipped.

What we need is to teach generate_base_implied_equalities_const() to
prefer an actual constant to pseudo-constants.  Will fix, thanks for the
example!

Thanks for nice explanation.

 

                        regards, tom lane



--
Rushabh Lathia